A known advantage of linear motors is to avoid using means for transforming rotary motion into movement in translation.
However, in linear motors, the primary portion includes a winding, and the secondary portion, whose length is equal to that of the primary portion plus its displacement stroke and can therefore be very long, includes either a winding of the cage type, when the linear motor is of the asynchronous type, or permanent magnets, when the linear motor is of the synchronous type. In both cases, such a motor is costly, especially if its stroke is long: for an asynchronous linear motor, the secondary winding must be cooled; and for a synchronous motor having magnets, magnets based on rare-earths are generally used, thereby making it too costly for a stroke of significant length; in addition, the magnets strongly attract any magnetic material and the motor must be closed inside a sealed chamber which is difficult to achieve and costly.
A hybrid linear stepper motor is also known. In such a motor, the primary portion comprises a magnetic circuit including teeth and slots along an air gap, and split in two by a magnet, each magnetic half-circuit is provided with an electrical coil, both coils being fed by a current which is offset by 90.degree. from one coil to the other. The secondary portion is a magnetic circuit possessing teeth and slots. Such a motor is inexpensive but very bulky. A continuous flux passes through the stator laminations and is amplitude modulated by passing via the teeth, but the flux is never reversed. The force density per unit area of this type of motor is about one third of that of a motor where the flux is reversed in the laminations.